Small cleanup of MachineLICM.cpp

Small cleanup of MachineLICM.cpp

Specifically:
- Calculate the loop pre-header once at the stat of HoistOutOfLoop, so:
  - We don't-DFS walk the MachineDomTree if we aren't going to do anything
  - Don't call getCurPreheader for each Scope
- Don't needlessly use a do-while loop
- Use early exit for Scopes.size() == 0

No functional changes intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@228350 91177308-0d34-0410-b5e6-96231b3b80d8